Coal gasification is the process to convert carbonaceous materials into syngas primarily containing carbon monoxide and hydrogen. Fluidized bed reactors are often commonly used to gasify carboneous materials such as coal. An example of such a gasifier is provided in U.S. patent application Ser. No. 13/532,769 (hereinafter referred to as an “SES Gasifier”). A characteristic of such a gasifier is that the operating temperature of the gasifier is just below the melting point of the ash, and as such these gasifiers are called non-slagging gasifiers.
As shown in FIG. 1, feed stock which can be a variety of carbon containing materials is usually delivered to the gasifier's fluidized bed region 1 as small particles. As their carbon content is depleted, the small particles coalesce due to collision and fusion in the fluidized bed region 1, and become heavier than those particles whose carbon content is not yet depleted. The carbon-depleted ash particles then fall by gravity as solid ash particles from the fluid bed region of the gasifier, into an ash discharge device 4, which is vertically arranged underneath the fluid bed.
This discharge device 4 generally comprises a centrally located pipe 6 (“center jet pipe”) at the bottom of the dense phase region of the gasifier. The center jet pipe is used to introduce jet gas which contains oxygen-rich gas at the bottom of the dense phase region, so as to form a high temperature region. This region is relative rich in oxygen, and the combustion reaction of the coal material improves carbon conversion.
The discharge device 4 may also be configured without the use of a center jet pipe 6, so long as the discharge device 4 is located at the bottom of the gasifier dense phase through which carbon depleted solids are removed and partially cooled.
Ash particles also undergo some separation between coarser and finer particles in the ash discharge device 4 integrated with a classifier 5. A gas stream, moving upwards through the discharge device, is often used to separate the ash particles, re-entraining those lighter and/or smaller particles whose carbon content is not yet depleted and returning them back into the reaction region, while allowing the heavier, carbon depleted ash particles to fall through. The gas stream also serves to cool the central pipe if the discharge device is configured with a central pipe. The gas flow in the discharge device generally operates at a higher gas velocity at least in the upper portion than the gasifier fluid bed and in some portions it may operate at a lower superficial gas velocity.
The gas flow needed for the ash discharge device to separate the ash particles, being steam or other gases, or a mixture thereof is usually in excess of the amount chemically needed for the gasification reaction and is at a lower temperature than the gasifier, hence it imposes an energy load on the gasifier which reduces the cold gas efficiency.
Referring to FIG. 2 as an example, in the SES U-gas technology, fine solid particles (“fines”) also exit the gasifier from the top with the raw product syngas. These fines are captured, both as part of the process for cleaning the syngas, and for recycling the carbon content of the fines, usually via cyclones and/or filters 8.
Fines from the cyclone and filters downstream of the syngas cooler are recycled into the gasifier to boost carbon conversion and improve the amount of syngas produced per unit input of coal. Currently, these fines are transported directly into specific regions of the fluid bed 1 using dense phase or dilute phase conveying of solids by gas. The specific region is usually the higher temperature central region of the gasifier well below the top of the dense phase bed of fluidized material.
The gas used for the transportation purposes is referred to as the transport gas. The amount of the transport gas used to transport these solids increases as gasifier operating pressure increases. The transport gas necessarily has a lower temperature than the gasifier reaction region. Accordingly, it imposes a penalty on gasifier efficiency and increases oxygen consumption.
In addition, the increase in gas rate leaving the gasifier per unit of H2 and CO produced increases the size of the gasification system equipment, such as cyclones, syngas cooler, and filters downstream of the gasifier. With high ash coals and at high pressures such as 40 bar, the fines transport gas could be as much at 5-10% of the total dry gas leaving the gasifier.
Therefore there is a need in the art for an improved gasification system and process wherein the penalty on gasifier efficiency imposed by the transport gas is decreased.